Human cells which have undergone neoplastic transformation may express an array of cell surface structures not found on their normal conterparts. Over the past several years, a number of monoclonal antibodies to human cell surface antigens have been produced detecting antigens controlled by the major histocompatibility complex as well as other cell surface structures. These sera were reacted with human tumor cells growing in tissue culture. The enzyme linked immunosorbent assay (ELISA) technique was modified in order to detect antigen-antibody reactions on surface adherent tumor cells. The studies thus far indicate that the assay system is applicable to the examination of cell surface determinants detected by these monoclonal antibodies. This technique will be applied to the assessment of antigens on normal epithelial cells and cells treated with chemical carcinogens.